An electrochemical capacitor (electric double-layer capacitor) is a promising electric energy storing device, which allows frequent charging and discharging, because unlike secondary batteries that store electric energy by a chemical reaction induced at an electrode, the electrochemical capacitor stores electric energy by storing charge with ionic molecules. Therefore, the electrochemical capacitor will have less deterioration caused by charging and discharging, with only a small degree of deterioration of an electrode and deterioration of ionic molecules of the electrolyte at around withstand voltage. Accordingly, the electrochemical capacitor can allow about a few millions cycles of charging and discharging.
The electrochemical capacitor however has low withstand voltage, and the maximum chargeable voltage of approximately 3V, and therefore it is connected series when the charging voltage is high. When the parallel connection and series connection of the electrochemical capacitor are repeated during a cycle of charging and discharging, a chargeable capacity of the electrochemical capacitor reduces.
Accordingly, there is a need for improving energy density, rapid charging and discharging properties, and durability of an electrochemical capacitor.
In the electrochemical capacitor, an electrolyte acts as a carrier of charge in an electrolyte solution. Depending on a type of an electrolyte used, internal resistance and electrostatic capacitance of the capacitor vary. Generally, internal resistance of a capacitor can be reduced by incorporating polyvalent cations in a molecular of an electrolyte. Moreover, a small molecular weight of the electrolyte is economically advantageous as the production thereof is easy, but such electrolyte of small molecular weight has low ability for preventing solidification with low temperature of the electrolyte solution, and may be evaporated when used over a long period.
In order to reduce internal resistance of a capacitor, and solve the problem associated with a small molecular weight of an electrolyte, there is proposed an electrolyte having quaternary ammonium cations at both terminals of a polymer chain.
As for the electrolyte having quaternary ammonium cations at terminals of a polymer chain, there have been discussed an electrolyte having quaternary ammonium cations in the back bone, as well as the terminals of the polymer chain (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-151360), and an electrolyte whose polymer chain is an alkyl chain (see, for example, JP-A No. 2002-93665).
The electrolyte having quaternary ammonium cations in the back bone as well as the terminals of the polymer chain tends to cause decomposition by hydrolysis upon application of high voltage, and therefore there are problems that voltage to be applied is limited, and that the production cost thereof is high, as the synthesis thereof include many stages of chemical reactions.
The electrolyte a molecular chain of which is an alkyl chain has a problem that it has poor solubility and cannot increase a concentration thereof in a solution, which reduces internal resistance of a capacitor.